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Date: Thu, 8 May 2008 16:33:40 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
cc: Ingo Molnar <mingo@elte.hu>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Andi Kleen <andi@firstfloor.org>, Matthew Wilcox <matthew@wil.cx>,
       LKML <linux-kernel@vger.kernel.org>,
       Alexander Viro <viro@ftp.linux.org.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [patch] speed up / fix the new generic semaphore code (fix AIM7
 40% regression with 2.6.26-rc1)
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On Fri, 9 May 2008, Alan Cox wrote:
> 
> I don't think fixing n_tty is now a big job if someone wants to take a
> swing at it. The driver write/throttle/etc routines below the n_tty ldisc
> layer are now BKL clean so it should just be the internal locking of the
> buffers, window and the like to tackle.

Well, it turns out that Ingo's fixed statistics actually put the real cost 
in fcntl/ioctl/open/release:

    310  down <= lock_kernel <= sys_fcntl <= system_call_after_swapgs <
    332  down <= lock_kernel <= vfs_ioctl <= do_vfs_ioctl <
    380  down <= lock_kernel <= tty_release <= __fput <
    422  down <= lock_kernel <= chrdev_open <= __dentry_open <

rather than the write routines. But it may be that Ingo was just profiling 
two different sections, and it's really all of them.

			Linus